A sonogram, sonograph, or spectrogram is a well-known spectrum display technique in speech research, having been used for decades to analyse utterances. A sonogram shows an overview of the spectrum of several seconds of sound. This enables the viewer to see general features such as the onset of notes or phonemes, formant peaks and major transitions. The sonogram representation has also been employed as an interface for spectrum editing. A sonogram represents a sound as a two-dimensional display of time versus "frequency+amplitude". The vertical depicts frequency (higher frequencies are higher up in the diagram) and shades of gray indicate the amplitude, with dark shades indicating greater intensity. (Source - Curtis Roads (1996). The Computer Music Tutorial. Boston: MIT.)